It is well-known that a surfactant organizes silica into a variety of meso-porous structures through the mediation of electro static force, hydrogen bond, covalent bond or van der Waals interactions. The pore wall of meso-porous structural silica prepared by using the surfactant as a matrix is amorphous. This method of forming meso-porous structures has been tried with non-silica oxides. Due to sporadic success, this trial is extended to a material which applies the physical characteristics such as electron transfer or magnetic interactions.
Regarding oxides such as TiO2, ZrO2, Al2O3, Nb2O5, Ta2O5, WO3, HfO2, SnO2 or metal oxides including mixed oxides such as SiAlO3.5, Si2AlO5.5, SiTiO4, ZrTiO4, Al2TiO5 or ZrW2O8, there is a paper reporting that a material which has a meso porous structure having thermally stable and ordered pores of relatively large size (up to 140 Å, 14 nm) and which can be broadly applied is obtained [NATURE, Vol. 396, 152-155 (1998):
[hereinafter shortened to Paper A].
In that paper, the following items are reported. Namely, the product original material is prepared by dissolving 1 g of polyalkyleneoxide block copolymer [HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2OCH2CH2O)20H, hereinafter shortened to EO20PO70EO20; Pluronic P-123, product of BASF] into 10 g of ethanol (EtOH). To this solution 0.01 mol of the respective inorganic chloride precursor is added with vigorous stirring for half hour so as to obtain a sol solution which is poured into an open Petri dish and placed in the atmosphere of 40° C. for 1 to 7 days to gel. During the gelling process, the inorganic chloride precursor (chloride of transition metal) is hydrolyzed and polymerized, and organized to a metal oxide network structure. Alternatively, a material to be coated, such as a substrate, is dipped into the sol solution such that a coated film is formed on the surface of the substrate and a gel product is prepared according to the above mentioned reaction. In this case, it is reported that the appearance of all such made products are transparent except the appearance of product derived from WO3 is dark blue.
The obtained gel product is then heated at 400° C. for 5 hours and the surfactant is removed by this calcination process, and ZrO2 having meso structure corresponding to the matrix of EO20PO70EO20 is obtained.
The authors of the paper surmise that the formation of the meso structure is by way of a mechanism which combines the self organization of the block copolymer and the complexation of the inorganic species (segment of alkylene oxide forms crown ether type complex).
Further, the paper reports that although the pore wall of meso porous silica is amorphous, the above mentioned pore wall of the meso structure has a structure possessing a crystalline domain of nano size (up to 30 Å, 3 nm) in a comparatively thick amorphous wall.
Furthermore, in the paper, the sequence of atomic (lattice surface) and chemical components (in which the metal oxide is governable) of the net work structure are considered by an X ray diffraction method (XRD) or by an energy dispersive type X ray spectroscopic method (EDX), and the ordered semi crystalline inorganic stereo network characteristic of the meso structure is investigated by a conventional transmission electron microscope. The authors explain that the pore wall has a structure characterized in that the nano crystalline structure smaller than 30 Å is embedded in an amorphous matrix.
Still further, regarding the pores, by considering the N2, adsorption/desorption isothermal characteristic, the type of pore, porosity (smaller than approximately 50%) and pore size (50-140 Å) are investigated. According to the investigation by the inventors of the present invention, since the pore wall described in the paper is composed of amorphous matrix, it is obvious that the pore wall has problems in that the thermal or hydrothermal stability is comparatively small and the walls physically fragile.
The inventors of the present invention determined that the method to resolve the above mentioned problems which a meso size structure composed of well known metal oxide has is to make the metal oxide having a crystallized pore wall.
Therefore, the object of the present invention is to provide a meso porous transition metal oxide characterized in that the pore wall maintains a crystalline structure over the whole primary particles, and to provide a method for preparation of such meso porous transition metal oxide having the crystalline structure.